LibWeb: Run setTimeout() and setInterval() callbacks as HTML tasks

We now invoke DOM timer callbacks via HTML tasks. This brings callback
sequencing closer to the spec, although there are still many
imperfections in this area.

Userland/Libraries/LibWeb/HTML/EventLoop/Task.h

@@ -25,6 +25,7 @@ public:
         IdleTask,
         PostedMessage,
         Microtask,
+        TimerTask,
     };
 
     static NonnullOwnPtr<Task> create(Source source, DOM::Document* document, Function<void()> steps)